Pressure sensitive adhesives based on non-thermoplastic hydrocarbon elastomers such as natural rubber, butyl rubber, synthetic polyisoprene, ethylene-propylene, polybutadiene, polyisobutylene, or styrene-butadiene random copolymer rubber, are known in the art. Typical processing of such adhesives includes masticating the elastomer on a two roll mill or in a Banbury type internal mixer, dissolving the elastomer and other adhesive components in a hydrocarbon solvent, coating the solution onto a backing, and drying the coated product to remove the solvent. This technology is discussed in Handbook of Pressure Sensitive Adhesive Technology, D. Satas (ed.), p. 268. Van Nostrand, N.Y., (1989). This solvent process has the disadvantages of being labor intensive and having low production rates. Moreover, such solvent based processes have become increasingly undesirable for use in making adhesive tapes because of increasing environmental regulations throughout the world.
Another processing method, sometimes used when a relatively thick adhesive layer is desired, includes masticating the elastomer as described above, blending the rubber and other adhesive components in an internal mixer such as a Banbury mixer, and calendering the solid adhesive onto a backing using a three or four roll calender stack. The calendering process does not use solvent but does require very expensive equipment. Additionally, this process is snow, and is only economical when adhesive coatings greater than about 2 mils (51 mm) thick are desired. An application of the calendering process is discussed in U.S. Pat. No. 2,879,547 to Morris.
Environmental considerations, lower initial capital investments, potentially higher production rates, and lower processing costs have led to accelerated interest in the use of continuous hot melt compounding and extrusion coating of thermoplastic adhesive compositions. Typical elastomers employed in this technique are "thermoplastic" elastomers of the block copolymer type, including for example, styrenic-diene block copolymers. Such materials exhibit a sharp reduction in viscosity at temperatures above 100.degree. C. where the styrene domains soften. Upon cooling, the domains reform and the material regains its rubbery character and cohesive strength. Illustrative teachings of adhesive formulations and processes of this type are found, for example, in U.S. Pat. No. 3,932,328 to Korpman, U.S. Pat. No. 4,028,292 to Korpman, and U.S. Pat. No. 4,136,071 to Korpman. The technology is further discussed in Handbook of Pressure Sensitive Adhesive Technology, pp. 317-373, D. Satas (ed.), Van Nostrand, N.Y., (1989).
Hot melt pressure sensitive adhesives based on these thermoplastic elastomers have found wide acceptance in the packaging, label, diaper closures, and masking tape markets. The adhesive properties of pressure sensitive adhesives made from thermoplastic elastomers, such as those mentioned above, differ from those of adhesives based on non-thermoplastic hydrocarbon elastomers, and are undesirable for many tape applications.
Hot melt extrusion of pressure sensitive adhesives employing non-thermoplastic hydrocarbon elastomers such as natural rubber has been shown. However, low molecular weight plasticizing aids such as processing oils, elastomer oligomers, waxes, or other materials defined and described as plasticizers in Dictionary of Rubber, K. F. Heinisch, pp. 359-361, John Wiley & Sons, N.Y., (1974), are used as major components in the adhesive formulations. These plasticizing aids ease processing but detract from the ability of the finished adhesive to sustain a load and are generally known to degrade adhesive performance.
Canadian Patent No. 698,518 to P. Beiersdorf & Co., discloses a solventless extrusion coating process for coating a PSA composition based on non-thermoplastic elastomers including natural and synthetic rubber, high molecular weight polyisobutylene and polyvinyl ether. The elastomer is pre-masticated and blended in a separate, batchwise operation using conventional rubber processing equipment such as a two-roll mill or a Banbury mixer. The preformed, compounded mixture is then fed to a heated single screw extruder and the molten coating is extruded onto a moving web. Plasticizing aids comprising up to 54% of the formulation are used. It is believed that these plasticizing aids are used to accommodate the coating difficulties normally associated with the extrusion of high viscosity elastomers.
Japanese patent application Sho 50-37692 to Fukugawa et al. discloses a similar process of pre-masticating mixtures of ingredients of pressure sensitive adhesives for 25 minutes, supplying the premasticated mixtures to a heated extruder, extruding the materials at 230.degree. C. onto a substrate, and curing the extruded materials by exposing them to electron beam radiation to enhance the cohesive strength of the adhesive and improve the bond to the backing. The materials described include non-thermoplastic elastomers of natural rubber and styrene-butadiene rubber (SBR). In the two examples utilizing natural rubber, the natural rubber was blended with a styrene-butadiene elastomer and a plasticizing aid. The plasticizing aid equaled about 87.5% of the total rubber charge, and no tackifiers were used. The non-natural rubber example included 25.8% plasticizing aid.
German provisional patent publication P-19 54 214.4 to Pyton AG discloses an extrusion process for the preparation of pressure sensitive adhesives which does not necessitate a separate pre-mastication step. A twin screw extruder is used to continuously compound and coat a formulation comprised of five different types of materials. Natural rubber and/or partially vulcanized rubber, latex, polybutene with a molecular weight between 70,000 and 200,000, and polyisobutylene with a molecular weight between 100,000 and 250,000 comprise the cohesive component. Four other classes of ingredients are required to accommodate this process. These other ingredients include low molecular weight (less than 15,000) polybutene and polyisobutylene or native bitumen, reactive and/or non-reactive resins, antioxidants, and various metal oxide fillers. No specific compositions are taught, but the levels of plasticizing aids such as bitumen, or the low molecular weight polyisobutylene or polybutene are reported to range from 10 to 20%.
U.S. Pat. No. 2,199,099 to Cunningham discloses that air and oxygen enriched gases can be used to facilitate the oxidative breakdown of natural rubber in an internal mixer to reduce the molecular weight of the rubber. A continuous hot melt extrusion process that employs the air-assisted oxidative break-down of natural rubber followed by the addition of tackifiers and phenolic resin vulcanizing agent to form a thermosettable adhesive is known. In this process the molecular weight of the natural rubber is reduced to such a degree that when the phenolic resin is added, the combination of the rubber and resin can be processed at a temperature below that at which vulcanization occurs.
U.S. Pat. No. 5,158,725 to Handa et al. disclose a process utilizing a twin screw extruder to compound non-thermoplastic elastomers with fillers, additives, oil, another elastomer, pigments and/or curatives. The components are fed at controlled rates through a feed opening or openings to mix the elastomer with the non-elastomer components as they are added to provide a mixture while controlling the melt temperature, pressure, and torque required by the extruder. It further discloses a computerized control system to automate the process. It is also described that the elastomer or elastomers and other additives are added and masticated along the length of the screw to control mix, temperature and final melt viscosity.
U.S. Pat. No. 5,539,033 to Bredahl et al. discloses a continuous hot melt process for preparing a non-thermosettable, pressure sensitive adhesive from tackified high molecular weight non-thermoplastic hydrocarbon elastomer or elastomers. The process described uses a twin screw extruder employing sequential feed ports and alternating conveying and mixing sections to masticate and mix the components. The elastomer or elastomers can be fed in a single section, for example, section 1, and initial elastomer mastication occurs in the absence of tackifier. However, it is also described that the elastomers may be added sequentially to different conveying and processing sections. It is further described that mastication is preferably carried out in the absence of materials which will lubricate the elastomer (for example, a plasticizing aid) and prevent reduction of its molecular weight but that the presence of small amounts of such materials is not precluded, provided that the amount present does not effectively reduce the rate of mastication.
It is desirable to improve known hot melt compounding and extrusion of non-thermoplastic hydrocarbon elastomers using a single elastomer base to make pressure sensitive adhesives having the properties needed for high performance PSA tapes, such as high temperature masking, and medical tapes. In general, multi-polymer based pressure sensitive adhesives are needed to provide shear resistance in differing temperature ranges and peel rates; these resulting multi-polymer based adhesives typically exhibit a shear resistance over a broader range of temperatures and peel rates which in turn allows the coated tape to perform over a wider range of environmental conditions.